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There are some rules to be followed when you are creating the files. These rules are specific to the File system you are using. If you are using DOS then the file system supports 11 characters long file names.
Eight bits are reserved for name of the file and 3 bits for file extension. The extension of the file associates the file with type of application in the computer example file1.txt is a text file Here .txt identifies the file as text file.
Windows operating systems support long file names. But there are some rules to be followed while giving the file names. You cannot use characters like ", *, <, >, :, ?, /, \, ½and " while creating the files. You need not specify the file extension in windows, as windows will make you choose the type of file from the available list.
You can always identify the file by looking at its extension. Look into the more info section to know what kind of file types are available in windows. 
